In a receiving device receiving a radio signal and processing the signal, a rectifier is widely used for signal detection. When the rectifier (rectifying circuit) is used for signal detection, it is possible to easily achieve radio communication of low power consumption. An example of the receiving device using the rectifier is what is called an RFID tag. The RFID tag rectifies a transmitted signal from a reader/writer to obtain operating power of the RFID tag itself and demodulates a received signal.
Such an RFID tag normally has a rectifying circuit using a transistor in which a gate and a source are connected. However, the rectifying circuit using the transistor is not able to receive a weak signal due to a threshold voltage which the transistor has. Accordingly, there has been proposed a sensitive rectifier which intermittently applies between the gate and the source of a transistor constituting the rectifying circuit a bias voltage substantially equal to the threshold voltage of the transistor. The proposed rectifier is able to reduce the influence of the threshold voltage of the transistor. Thus, when this rectifier is applied to a receiving device, it is possible to receive a weak signal.
However, the proposed rectifying circuit gives the bias voltage to the transistor by intermittently operating a switch, and hence generates switching noise. That is, there has been a problem that the receiving device including this rectifier is affected by the switching noise.